UNITED NATIONS (AP) — U.N. chemical weapons experts have delayed a trip to Syria because the United Nations and the Syrian government haven’t agreed on arrangements for the investigation of alleged chemical weapons use, a spokesman for the world body said Tuesday.

U.N. disarmament chief Angela Kane was continuing consultations with the Syrian government “with a view to reaching agreement as soon as possible on the modalities essential for cooperation to ensure the proper, safe and efficient conduct of the mission,” U.N. deputy spokesman Eduardo del Buey said.

Del Buey refused to say what specific issues were holding up the team’s departure.

“Obviously the devil is in the details, and the details are being worked out,” he said.

The expert team, led by Ake Sellstrom of Sweden, completed preparations for the visit over the weekend, del Buey said.

U.N. spokesman Martin Nesirky had said on Aug. 6 that preparations were expected to be completed “within the next days, following which the date of the mission in Syria will be announced.”

Del Buey said that “once the government of Syria confirms its acceptance of the modalities, the mission will depart without delay.”

The U.N. gave approval for the probe on July 31 following an “understanding” reached during meetings in Damascus between the Syrian government and the U.N., represented by Kane and Sellstrom, that three sites where chemical weapons were allegedly used would be investigated.

The team is expected to visit Khan al Assal, a village on the southwestern outskirts of the embattled city of Aleppo which was captured by the rebels in late July. The government and rebels blame each other for a purported chemical attack on the village on March 19 that killed at least 30 people.

The experts are expected to investigate two other incidents whose locations are being kept secret for safety and security reasons.

U.N. Mideast envoy Robert Serry told the Security Council last month that the U.N. has received 13 reports of alleged chemical weapons use in Syria.

The investigation team includes about 10 experts from the Organization for the Prohibition of Chemical Weapons, which is based in The Hague, and the World Health Organization, based in Geneva. Its mandate is to report on whether chemical weapons were used, and if so which ones, but not to determine the responsibility for an attack.

President Bashar Assad’s government initially asked the U.N. to investigate the Khan al Assal incident. Britain, France and the U.S. followed with allegations of chemical weapons use in Homs, Damascus and elsewhere.

The July 31 announcement of an investigation ended more than four months of behind-the-scenes talks, with Syria trying to limit the probe to Khan al-Assal and the U.S., Britain and France pressing for a broader investigation.